This invention relates generally to a voltage controlled oscillator (VCO) and more particularly to a voltage controlled oscillator having a plurality of MOS capacitors. The VCO has substantially linear tuning characteristics and an increased tuning range.
A local oscillator (LO) signal is required for receiving and transmitting in a wireless radio frequency (RF) communication device, such as a cellular telephone. A voltage controlled oscillator (VCO) is typically used in a phase-locked loop (PLL) to generate the LO signal. An example of the use of a VCO in a mobile station can be found in commonly assigned U.S. Pat. No. 5,471,652, entitled xe2x80x9cFrequency Synthesizer and Multiplier Circuit Arrangement for a Radio Telephonexe2x80x9d, by Jaakko Hulkko. The circuit arrangement disclosed therein is used for a radio telephone wherein a universal application is provided by phase locked circuits and a multiplier circuit is controlled by a processor of a mobile phone.
Another example of a VCO used in a mobile station can be found in commonly assigned U.S. Pat. No. 5,926,071, entitled xe2x80x9cMinimization of the Power Consumption in an Oscillatorxe2x80x9d, by Osmo Kukkonen. The oscillator""s output signal is detected as a DC voltage in a clamp/voltage multiplier circuit. The detected signal is supplied in a feedback loop to a field effect transistor (FET) controlling the oscillator""s current, whereby the FET controls the current in the main current path to a predetermined minimum value.
U.S. Pat. No. 5,764,109, entitled xe2x80x9cVoltage-Controlled Oscillator (VCO) Having a Voltage Derived from its Output to Tune its Center Frequencyxe2x80x9d, by Osmo Kukkonen relates to an electrically tunable voltage controlled oscillatory circuit, wherein the negative bias voltage (xe2x88x92Vcf) of a capacitance diode needed for tuning the center frequency of the oscillatory circuit is generated on the basis of an electric oscillating signal produced by the oscillatory circuit itself. The oscillating signal is used for generating a negative voltage with a clamp/voltage multiplier circuit and it is adjusted to a desired value with an adjustment circuit in which the values of the components can be permanently adjusted. Alternatively, the adjustment circuit may include an active component that can have an effect on the value of the negative bias voltage (xe2x88x92Vcf) during the use of the oscillatory circuit.
European Patent Application 0,599,505, entitled, xe2x80x9cTunable Resonance Circuit for a Voltage Controlled Oscillatorxe2x80x9d, by Jorma Liukkonen relates to a voltage-controlled oscillator (VCO) of a frequency synthesizer that can be controlled to expand the frequency range by a switch. Depending on the anode voltage of a diode, two voltage control ranges can be produced for the voltage controlled oscillator (VCO). This means that the VCO has two frequency ranges although the external voltage changes only in one range.
Reference may also be had to commonly assigned U.S. Pat. No.: 5,357,222, xe2x80x9cVoltage Controlled Component Including a Capacitive Diode Biased to Operate in the Linear Regionxe2x80x9d, by Seppo Hietala.
In oscillator applications, it is desirable to have a large tuning range and favorable noise performance. The gain of the VCO may be difficult to keep constant since it is dependent on the varactor tuning linearity. It is also desirable to have a substantially direct and linear relationship between the input voltage (control voltage) and the output frequency of the oscillator.
The linearity of the frequency tuning characteristics is typically achieved by using additional integrated circuit (IC) process steps to manipulate the capacitance characteristics of a device. Also, the effect of a non-constant VCO gain can be compensated by a feedback loop to the current of the charge pump. The tuning range can be increased dramatically by using two or more separate VCOs.
The power consumption of third generation and multi-mode phones is likely to increase compared to conventional mobile communications systems. Thus the operation and stand-by times of these phones will also likely increase. The need for power consumption and integrated circuit (IC) area reduction is evident.
One concern is that it is very difficult to simultaneously achieve a large tuning range and good phase noise performance. Additionally, the gain of the VCO is difficult to keep constant.
What is needed to advance the stale of the art is a voltage controlled oscillator that has improved linearity of the tuning characteristics and an enhanced tuning range with reduced phase noise.
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.
Accordingly, one embodiment of the present invention is directed a voltage-controlled oscillator that includes a resonator circuit and a voltage source. The resonator circuit includes a capacitive circuit. The capacitive circuit includes a plurality of analog voltage controlled capacitive elements coupled to one another. The voltage source is coupled to the resonator circuit and provides a voltage to the plurality of analog voltage controlled capacitive elements. Each of the plurality of analog voltage controlled capacitive elements is activated as a function of the voltage thereby increasing linearity of a frequency tuning voltage parameter of the resonator.
Another embodiment of the present invention is directed to a variable capacitor. The variable capacitor includes an input node and an output node, a plurality of analog voltage controlled capacitive elements and a control voltage. The plurality of analog voltage controlled capacitive elements are electronically coupled to one another. The control voltage input is coupled to a control voltage source and coupled in common to each of the plurality of analog voltage controlled capacitive elements. The control voltage input provides a control voltage to each of the analog voltage controlled capacitive elements, such that each of the plurality of analog voltage controlled capacitive elements is activated as a function of the control voltage to provide a substantially linear change in capacitance over a range of control voltages between the input node and the output node.
Yet another embodiment of the present invention is directed to a variable capacitor device that includes a first capacitive circuit, a diode circuit and a control voltage input. The control voltage input provides a control voltage to the first capacitive circuit and the diode circuit, such that the first capacitive circuit and the diode circuit are activated in a sequence that is a function of the control voltage.
Furthermore, the variable capacitor may include a bias voltage input that is coupled to the first capacitive circuit and the diode circuit. The bias voltage input provides a bias voltage to the first capacitive circuit and the diode circuit such that the first capacitive circuit and the diode circuit are activated as a function of the bias voltage. This biasing of the capacitive circuit and the diode circuit may optionally activate the first capacitive circuit such that the first capacitive circuit affects a tuning curve of the diode circuit where the derivative of the capacitance is not constant.
Yet another embodiment of the present invention is directed to a method of operating a voltage controlled oscillator that includes generating a control voltage and providing a capacitive circuit that includes a plurality of analog voltage controlled capacitive elements, coupled in parallel. Each of the plurality of analog voltage controlled capacitive elements is sequentially activated as a function of the control voltage to increase the linearity of a frequency tuning voltage parameter.
This invention also provides a mobile station that includes a frequency synthesizer comprised of a PLL that includes a VCO. The frequency output of the VCO is tuned with a resonator that comprises a variable capacitance. The variable capacitance is constructed from a plurality of voltage controlled capacitive elements coupled in parallel. Also provided is an activating means for sequentially activating individual ones of the plurality of voltage controlled capacitive elements as a function of an input analog control voltage and a bias network. The result is an increase in the linearity of a frequency tuning curve of the VCO. The analog control voltage is sourced by a PLL phase detector. The bias network comprises a resistive network coupled to the voltage controlled capacitive elements, where the resistive network is coupled to one of a fixed or a variable bias voltage. The bias voltage may be established by one of an operational mode or frequency selection logic of the mobile station. At least one fixed capacitive element can be provided that is switchably coupled with at least one voltage controlled capacitive element to provide a plurality of discrete frequency tuning ranges.